Complaint Review: ADT home security

 In April I called ADT to install service midway through the service I came upstairs from my basement to find the ADT tech sifting through my personal belongings on a stand he claimed he "dropped a screw" which was odd because nothing he was doing involved him being where he was doing what he was doing I asked him to leave so he went downstairs and cut all of my wires. I called ADT and complained so for some reason I obliged to have them send out another tech to finish the job a day or two later I called and cancelled the service this was in April it's not January and I got a letter from their billing dept which the adt rep said they are not affiliated and are a 3rd party but the 3rd party said they are apart of ADT they are hitting me with a $1358.06 bill ??? I cancelled within the alotted time which they only give you a 3 day window to cancel I told them I cancelled within the proper time frame only to be given a run around now they are telling me they will report me to the credit collection if I don't pay them what can I do ??